The present invention relates to volume sensors for determining the volume of a fluid in a container. More particularly, the present invention relates to volume sensors of the type adapted for the continuous computation of the volume of liquid remaining in a container.
In a variety of applications, it is desirable to be able to determine at any given moment the remaining volume of fluid in a container. Examples in the medical field range from determining the remaining reagent in a source container for use in automated clinical instrumentation to remaining saline or other fluid volume in an intravenous infusion.
A variety of means have been devised in the past for permitting an attending physician or staff member to estimate a remaining fluid volume. In the most basic form, containers have been provided with transparent walls and a graduated scale which is visible from the outside. However, visual observation of fluid level is not always practical, such as when the container is not readily locatable within the clinician's view. In addition, in some applications it may be desirable to have a more precisely quantitative indication of remaining fluid level or volume, or the fluid volume data may be desirably utilized to trigger electronics such as alarm limits, pumps or other devices which add and/or drain fluid from the container, or other data processing activities.
Thus, there have been devised a variety of electronic fluid level detectors in the prior art. For example, U.S. Pat. No. 4,063,457 to Zekulin, et al. discloses an ultrasonic sensing device for mounting within a storage vessel such as a bilge or railroad tank car. The sensing device comprises an elongate tubular body to be vertically oriented within the tank car, having a port at the lower end to allow fluid to enter the tube and rise to the level of fluid in the remaining tank. Spaced apart vertically within the tube are a series of pairs of piezo-electric transducers which serve as transmitters and receivers of ultrasonic energy. Because ultrasonic energy propagates more readily through liquid than through air, the system can determine which pairs of piezo-electric transducers are below the fluid level and which pairs of piezo-electric transducers are above the fluid level. Thus, each piezo-electric pair serves to determine simply the presence or absence of fluid at that level.
Another submersible probe-type liquid level indicator is disclosed in U.S. Pat. No. 3,163,843 to Dieckamp. In this device, mechanical vibrations are propagated axially from a transducer through the probe which extends vertically within the fluid to be measured. Reflected transverse pulses are created at both the liquid surface and the submerged distal end of the probe, which are smaller in amplitude than the applied pulse. These reflected transverse pulses are propagated back to the transducer, and the waveforms of the various pulses are evaluated on an oscilloscope to determine the fluid level.
Another approach is disclosed in U.S. Pat. No. 4,144,517 to Baumoel. That patent discloses an externally mounted single transducer liquid level detector in which the presence or absence of liquid at a particular location of a tank or pipe is sensed through the wall of the container. The transducer may be secured to the outer surface of the container, such as by hand pressure, clamping or cementing. By comparing the rate of decay of the test signal following multiple wall reflections to known rate of decay values for filled and empty regions of the container, an output signal is produced which indicates the presence or absence of liquid in the container adjacent the transducer location.
Notwithstanding the foregoing, there remains a need for a volume sensor device which can be externally mounted to the container and which can provide remaining fluid volume data on a continuous basis from empty through full.